One of the essential components of a radiation imaging device is a radiation input converter. In most applications of position-sensitive radiation detection, conventional radiography, and digital radiography, a phosphor screen is used to convert the incoming radiation into visible light which is then detected on film, on a photodiode, or other position sensing device. Such phosphor screens are usually required to have high resolution and high output brightness characteristics. The brightness of the light output is, in part, a function of the thickness of the phosphor layer, which determines the amount or X-ray energy absorbed, and of the inherent phosphor efficiency in converting X-ray radiation into detectable light. However, as this layer is made thicker, the spatial resolution is decreased because light photons emitted in response to the absorption of X-ray photons or charged particles will scatter within the phosphor layer and emerge from the phosphor surface at points further away in the transverse direction. Such lateral light spreading is caused by two factors. First, the phosphor emits light photons isotropically from the point at which a radiation particle, such as the X-ray photon, is absorbed. Second, even light photons which are traveling more or less perpendicularly to the surface of the phosphor layer may be scattered in the lateral direction before they reach the surface. Thus, the actual thickness of the phosphor layer is a compromise between the desired high radiation absorption that may be obtained from thicker layers and the required or desired resolution which improves as the thickness of the layer is reduced. Clearly, it would be desirable to increase the thickness of the phosphor layer without degrading the spatial resolution. This can be accomplished by suppressing the lateral light spread within the layer. Certain techniques for forming small cracks perpendicular to the phosphor surface to limit light spread have been developed, and the characteristics of evaporated CsI(Tl) layers coupled to a Si:H photodiode have been disclosed, for example, in Assignee's U.S. Pat. No. 5,302,423, incorporated by reference herein.
The incorporation of cracks in the phosphor layer, which are essentially perpendicular to the surface of the phosphor layer, exhibit enhancement of the resolution of light emitted from the phosphor in response to incident X-rays. A phosphor "cell" is formed when a continuous coating of phosphor is surrounded by a boundary of cracks. An emitted photon from the interaction of the phosphor with an X-ray will be effectively contained within a phosphor cell due to the presence of a crack or gap in the phosphor. If the angle of incidence of the photon is sufficiently large, the photon will be reflected back into the phosphor cell by total internal reflection, thereby isolating the photons generated in one cell from an adjacent cell. Alternatively, if the space between adjacent cells is filled with either an opaque or reflective material, the photons generated within the cell will be absolutely contained within the cell.
One early approach to form a light-guiding structure in a luminescent layer was to deposit a thin scintillation layer of CsI on the substrate and impart thermal shocks to the CsI layer, producing cracks therein due to the different thermal expansion coefficients of the substrate and the CsI layer. Another light-guide structure fabrication method was made in which the substrate had a very thin Al.sub.2 O.sub.3 layer which is caused to form a mosaic of cracks due to imparting thermal shocks, forming small grooves on the substrate. This type of cracked or net-like mosaic substrate further enhances the columnar structure of the phosphor material deposited on it. However, a phosphor layer prepared by these processes has the following drawbacks: 1) the columns defined by cracks have an irregular structure, which decrease the light collimation and thus, decrease the resolution; and 2) it is difficult to ensure the reproducibility of the size or position of the randomly formed columns. For these reasons, CsI X-ray radiation layers (150-200 .mu.m thick) made by these methods have spatial resolution of only 4 to 6 line-pair/mm at the 10 percent level. It is desirable to generate a spatial resolution of at least 10 lp/mm for use in diagnostic radiography.
U.S. Pat. No. 4,209,705 teaches the columnar growth of CsI phosphor over a metal pattern which is deposited in the grooves between a random mosaic pattern formed on a substrate. The metal protrusions are used to intensify the image sharpness by restricting lateral illumination (spreading within the phosphor layer) via reflection off the metal pattern. Most of the collimating of the light occurs due to the total internal reflection within the columns of formed phosphor. The random mosaic pattern is an insulating layer formed on an aluminum substrate in one embodiment. In another embodiment, the mosaic forming layer is from a different plated material such as molybdenum oxide which is treated to form random cracks in the material. See also U.S. Pat. No. 4,236,077 for a similar approach.
"Enhanced Columnar Structure in CsI Layer by Substrate Patterning", Jing et at., IEEE Trans. Nucl. Sci., Vol. 39, No. 5 (Oct. 1992) discloses the use of photolithography to form a mesh-patterned substrate onto which a CsI phosphor layer is deposited. The columnar growth of the thallium doped CsI on the mesh pattern causes the initiation of a crack formed on the ridges of the mesh pattern, and the deformation stress controls further growth to propagate the crack. In this teaching, the benefit of initiating the growth over a ridge serves to enhance the parallel columnar growth of the phosphor, as it is shown that without the presence of ridges the phosphor grows with discontinuous random cracks oriented at a multiplicity of angles relative to the substrate. The width of the cracks formed are on the same order as the width of a discontinuity between adjacent columns in the structure. If the CsI layer is allowed to grow far enough, the cracks vanish at the top of the layer at about 450 microns in height.
U.S. Pat. No. 4,437,011 teaches the use of phosphor seed particles which are vapor deposited onto a substrate, which serve as nucleation sites for the subsequent deposition of additional phosphor which will grow as a series of columns vertically from the seed, with a crack or discontinuity between adjacent columns corresponding to the placement of neighboring seed particles. To permit a continuous layer to be formed on the surface of the crack separated columns, another phosphor layer is deposited over the columns in such a manner so as to form a continuous film for subsequent deposition of a transparent conductive layer for conversion of the light to photoelectrons.
E. P. Application No. 0,175,578 teaches the use of a phosphor which does not contain a binder to improve the response to irradiation. Mention is made of the advantage of having the phosphor align itself into discrete blocks, either formed on a PET sheet, or formed by treating an oxide surface.
U.S. Pat. No. 4,011,454 discloses a phosphor screen for converting X-rays to light which includes a large number of discrete columns of the phosphor material, such as doped CsI, with the spaces therebetween preferably filled with a reflective substance which itself may be a phosphor (e.g., Gd.sub.2 O.sub.2 S or La.sub.2 O.sub.2 S). Because of the resulting inhibition of lateral spread of light within the phosphor screen, it may be made thicker than conventional screens while achieving at least as high a resolution and contrast, thereby increasing brightness (and thus requiring lower X-ray "doses"). The patent also discloses a method for making the screen which includes using a patterned substrate and wide-angle vapor deposition (as in a hot-wall evaporator) so as to deposit the phosphor only on the raised portions of the substrate.
In the method of '454, a pattern is formed on the substrate to be used for deposition of the phosphor. The pattern used generally has a "checkerboard" pattern of straight-walled, square-based mesas separated by slots or grooves typically 100 microns in width. This substrate is then placed in a specially designed vacuum evaporation apparatus that is equipped with heated walls and a heated plate which serves as a shield placed between the evaporation boat and the substrate. When the wails and shield of the system are heated sufficiently, any phosphor particles in the vapor phase will reflect from, rather than stick to, these heated surfaces. In this manner, the phosphor, which is rapidly evaporated from the boat, will preferentially approach the deposition surface from a (somewhat controllable) large angle of incidence. As phosphor particles intercept the cooler surface of the substrate, deposition occurs preferentially on the raised portions of the mesas since the large angle of incidence shields the bottoms of the grooves from deposition. In this way, the mesas grow in height, remaining separated by a gap. As the deposition continues, the tops of the mesas begin to increase in lateral dimensions, thereby beginning to close off the intervening gaps between them, and the deposition must be halted to permit a high temperature annealing step. Annealing of the phosphor serves to densify the structure, essentially regenerating the entire gap width at which point the deposition steps can be repeated until the final thickness of the phosphor is obtained. In this process, the phosphor is deposited at a rapid deposition rate of about 50 microns per minute, and the annealing step takes place at an elevated temperature of about 450.degree. C.
U.S. Pat. No. 5,171,996 and PCT Publn. No. WO 93/03496 disclose a method and apparatus for producing separated columns of scintillation material offering improved spatial resolution. In this method, a pattern of vertical wailed ridges is formed on a substrate and the subsequent deposition of a phosphor over this surface is claimed to form discrete columns of phosphor separated by spaces which can be filled with a material which absorbs light. In this method, the deposition of a wide range of scintillation materials is claimed to form columns where the separation gap between columns is maintained until the columns reach a height not more than 50 times the lateral width of the vertical wailed ridges.
U.S. Pat. No. 5,153,438 discloses an electronic X-ray imaging array made by combining a two-dimensional photosensitive array with a structured scintillator array, having a common array pattern and suitable alignment marks thereon, by coding them face-to-face in alignment for direct coupling of X-ray luminescence from the scintillator array to the photosensitive array.
In view of the foregoing art, it is apparent that what is needed in the industry is a technique whereby a phosphor screen used for converting X-rays can be easily fabricated so that phosphor cells are associated with a single sensor and are isolated from adjacent sensor elements. In particular, it would be desirable to form this phosphor screen in a manner which prevents the substrate onto which it is attached from being subjected to any extreme conditions such as high temperatures which could potentially damage the substrate, since this substrate may be a semiconductor device used for the conversion of photons emitted from the phosphor to an electrical signal.